There are a number of devices, such as an optical spectrometer, which utilize a multi-element optical detector, such as an array of photodetectors, to detect and/or measure the features of an optical spectra. An important factor in their suitability for this purpose is the degree of focus achieved, in the plane where the features of the spectra are detected and measured. Thus, it is important in the manufacture and routine adjustment of these optical devices that the focus be optimized.
Optical devices, such as an optical spectrometer, have been generally focused by placing a series of photographic plates in the focal plane, and exposing each at a different focus setting. When the plates are developed, the best focus setting is chosen, based on the sharpness of the spectral lines. The assessment of sharpness is usually a matter of visual interpretation, combining subjective impressions of the narrowness and intensity of the spectral lines.
For optical devices, such as scanning spectrometers, where a spectrum is produced in the form of a strip-chart or in the form of a computer record of many different readings, a similar method can be used. A section of spectrum is examined after each change of the focus setting until an optimum is found. However, for this type of device a more objective measure of focus than visual interpretation can be used. The following method can be carried out either manually or automatically to measure the quality of focus:
1. The plot of the spectrum is examined to find a chosen peak. If there is more than one spectral peak, there needs to be a secondary rule for choosing a certain one. Such a rule might be to take the peak with the largest maximum intensity, or the peak closest to a certain wavelength. When focus is far from optimum, combinations of several peaks may be mis-interpreted as the most intense peak. If the wavelength scale is not precisely known, there may be difficulties in choosing the peak of interest.
2. Once a peak is chosen, it is necessary to examine the regions adjacent to the peak, and to choose two baseline points.
3. Now the sharpness can be measured, either as the height of the peak above baseline, or as the peakwidth of the peak, at one-half of the peak height above baseline for example. The first measure suffers if the peak intensity is varying. The second measure suffers, if the baseline points are affected by smaller peaks lying near the chosen peak. Both measurement techniques suffer if the wrong peak is chosen, or if the peak used for the measurement changes during the test. Both measurement techniques suffer if the measurement peak is not much larger than neighboring peaks in the spectrum.
Multi-element optical detector arrays can measure many points in a spectrum at the same time. Therefore, it is possible to use the resulting series of measurements to perform the above method. However, there are problems. In some cases, the size of the individual element of the detector array is nearly as big, or bigger, than a spectral line. In such a case, the relative location of the spectral line with the pattern of detector element causes a change in the measure of focus. This undesired change is known as the relative registration problem. If the measurement of focus is automated, other problems may occur. For example, it becomes difficult to separate out the effects of intensity changes and of other changes in the spectrum, from changes in focus.
Designers of photographic cameras have developed several ways to auto-focus the lens of the camera. Many of these auto-focusing techniques have common features in that they use a plurality of detectors and there is a separation of the incident light into two or more paths through the optics. The pattern of the multiple light beams on the detector array is then sensed. However, for many types of devices, such as spectrometers, these techniques cannot be easily applied For these devices it is preferred to use measurements of spectral structure of the light to measure focus, rather than multiple optical paths and it is important that the light path not be altered for proper operation of the device.